Chapter 19-Process of Evolution

19.1 Evolution in a Genetic Context

• population-all the members of a single species occupying a particular area at the same time
• microevolution-evolution that occurs within a population
• gene pool-various alleles at all the gene loci in all individuals
• population genetics-study of gene frequencies and their change within a population
• sexual reproduction alone cannot bring about a change in allele frequencies
• Hardy-Weinburg law-an equilibrium of allele frequencies in a gene pool (p² + 2pq + q²) will remain in effect in each succeeding generation of a sexually reproducing population as long as there are:
1. no mutations: allele changes do not occur, or changes in one direction are balanced by changes in the opposite direction
2. no gene flow: migration of alleles into or out of the population does not occur
3. random mating: individuals pair by chance and not according to their genotypes or phenotypes
4. no genetic drift: the population is very large, and changes in allele frequencies due to change alone are insignificant
5. no selection: no selective agent favors one genotype over another
   
• this usually never happens; therefore evolution has occurred
  
• a change in allele frequencies results in a change in phenotype frequencies
• industrial melanism-increased frequency of darkly pigmented (melanic) forms in a population when soot and pollution make lightly pigmented forms easier for predators to see against a pigmented background
• the conditions that cause a deviation from the Hardy-Weinburg equilibrium are mutation, gene flow, nonrandom mating, genetic drift, and natural slection (only element resulting from adaptations to the environment)
• mutations are the raw material for evolutionary change
• once alleles have mutated, certain combinations of several alleles might be more adaptive than others in a particular environment
• gene flow-gene migration; movement of alleles between populations by migration of breeding individuals
• there can be constant gene flow between adjacent animal populations due to migration
• gene flow can increase variation with a population by introducing new alleles due to natural selection and genetic drift
• gene flow among populations can prevent speciation
• nonrandom mating-breeding/mating between relatives to a greater extent than chance
• inbreeding doesn't change allele frequencies, but it changes the proportion of heterozygotes and increases the proportions of homozygotes at all gene loci
• in human populations, inbreeding increases the frequency of recessive abnormalities in the phenotype
  
• assortative mating-when individuals tend to mate with those that have the same phenotype with respect to some characteristic
• assortative mating causes the population to subdivide into two phenotypic classes, when there is reduced gene exchange, so homozygotes increase and heterozygotes decrease
• sexual selection-when males compete for the right to reproduce and females choose to mate with males that have a particular phenotype (e.g. peacocks)
• genetic drift-changes in allele frequencies of a gene pool due to chance
• larger populations suffer less sampling errors than smaller populations
• when genetic drift leads to a loss of one or more alleles, other alleles over time become fixed in the population
• genetic drift is a random process
• bottleneck effect-if a species is subjected to near extinction because of a natural disaster/overharvesting/habitat loss, so it's as if most of the population has stayed behind and only a few have passed through the neck of a bottle
• founder effect-example of genetic drift in which rare or combinations of alleles occur at a higher frequency in a population isolated from the general population

19.2 Natural Selection

• natural selection-the process that results in adaptation of a population to the biotic and abiotic environments
• the biotic environment includes organisms that seek resources through competition, predation, and parasitism
• the abiotic environment includes weather conditions dependent chiefly upon temperature and precipitation
• relative fitness-the fitness of one phenotype compared to another
• most traits on which natural selection acts are polygenic and controlled by more than one pair of alleles located at different gene loci; the range of phenotypes' frequency distribution resembles a bell-shaped curve
• directional selection-when an extreme phenotype is favored and the distribution curve shifts in that direction; this can occur when a population is adapting to a changing environment
• stabilizing selection-when an intermediate phenotype is favored which can improve adaptation of the population to those aspects of the environment that remain constant; individuals near the average are favored
• disruptive selection-two or more extreme phenotypes are favored over any intermediate phenotype
• a population always shows genotypic variation
• the maintainence of variation is beneficial because populations with limited variation may not be able to adapt to new conditions and thus become extinct
• only alleles that are exposed (phenotypic difference) are subject to natural selection
• in diploid organisms, this makes the heterozygote a potential protector of recessive alleles that otherwise would be weeded out of the gene pool
  
• in a changing environment, the recessive phenotype may then be favored by natural selection
• when the ratio of two or more phenotypes remains the same in each generation, it is called balanced polymorphism

19.3 Speciation

• speciation-the splitting of one species into two or more species or the transformation of one species into a new species over time
• species-group of similarly constructed organisms capable of interbreeding and producing fertile offspring; organisms that share a common gene pool; taxon at the lowest level of classification
• whenever reproductive isolation develops, speciation has occurred
• allopatric speciation-variations due to different mutations, genetic drift, and directional selection build up, causing first postzygotic adn then prezygotic isolation to occur
• sympatric speciation-a population develops into two or more reproductively isolated groups without prior geographic isolation
• adaptive radiation-the rapid development from a single ancestral species of many new species, which have spread out and become adapted to various ways of life